Re: Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit?

[Michael S. Tsirkin]

On Wed, Aug 05, 2020 at 01:11:07PM +0100, Stefan Hajnoczi wrote:
> On Thu, Jul 30, 2020 at 07:46:09AM +0000, Yajun Wu wrote:
> > I'm doing iperf test on VIRTIO net through vhost-user(HW VDPA).
> > Find maximal acceptable tx_queue_size/rx_queue_size is 1024.
> > Basically increase queue size can get better RX rate for my case.
> > 
> > Can we increase the limit(VIRTQUEUE_MAX_SIZE) to 8192 to possibly gain 
> > better performance?
> 
> Hi,
> The VIRTIO 1.1 specification says the maximum number of descriptors is
> 32768 for both split and packed virtqueues.
> 
> The vhost kernel code seems to support 32768.
> 
> The 1024 limit is an implementation limit in QEMU. Increasing it would
> require QEMU code changes. For example, VIRTQUEUE_MAX_SIZE is used as
> the size of arrays.
> 
> I can't think of a fundamental reason why QEMU needs to limit itself to
> 1024 descriptors. Raising the limit would require fixing up the code and
> ensuring that live migration remains compatible with older versions of
> QEMU.
> 
> Stefan

There's actually a reason for a limit: in theory the vq size
also sets a limit on the number of scatter/gather entries.
both QEMU and vhost can't handle a packet split over > 1k chunks.

We could add an extra limit for s/g size like block and scsi do,
this will need spec, guest and host side work.

-- 
MST